The present invention relates to photographic apparatus and in particular to a method and device for determining when an objective of the photographic apparatus has properly focused an image in a predetermined plane.
It has already been proposed to detect proper or improper focusing from the knowledge that various factors such as contrast, spatial frequency, and luminance of an object to be photographed will vary in accordance with the extent of precision of the focusing. One of the known methods utilizes photoconductive elements such as cadium sulfide elements having specific contrast-responsive characteristics. Another type of known system utilizes a photoelectric output signal obtained by oscillating or rotating an optical member in a mechanical manner. Also it is known to utilize two outputs from two sets of photo-detecting members arranged at different positions, as in known so-called coupled range-finder cameras.
However, the method utilizing the contrast-responsive characteristics of cadium sulfide elements is disadvantageous in that it is difficult to prepare these elements in such a way that they have a uniform photoresponsive characteristic. Thus the output variations of such photoconductive elements before and after the properly focused setting is too low for achieving an exact detection insofar as detecting of proper focusing at a relatively low luminance intensity is concerned.
With respect to the second method utilizing a mechanically movable oscillating or rotating optical member, there is the considerable disadvantage that mechanically movable parts are of considerable size so that the desired miniaturization of a camera or other photographic apparatus cannot be achieved, and at the same time such constructions have an undesirably great power requirement.
The third of the above solutions utilizing two outputs from two sets of photo-detective members is of considerable disadvantage in that exchange of lenses with such systems is extremely difficult to carry out.